1. Field of the Invention
The present invention relates to an exhaust purification system for an internal combustion engine and a control method of the exhaust purification system.
2. Description of the Related Art
As described in JP-A-H6-307271, an oxygen concentration sensor is disposed upstream of a catalytic converter in the engine exhaust system to detect an oxygen concentration in the exhaust gas and estimate the optimum air-fuel ratio based on the detected oxygen concentration.
When the air-fuel ratio is leaner than the stoichiometric ratio (i.e., when the air-fuel ratio increases), the oxygen concentration in the exhaust gas increases more noticeably. This ensures a more accurate estimation of the optimum air-fuel ratio. However, when the air-fuel ratio is richer than the stoichiometric ratio, the oxygen concentration in the exhaust gas decreases to almost zero and then shows less noticeable change than the air-fuel ratio does. This makes the accurate estimation of the optimum air-fuel ratio more difficult.